The present invention relates to an arrangement in cylinder locks of the kind which comprise a plug or cylinder which can be rotated relative to a housing embracing said plug, when the correct key is inserted, which plug and which housing are provided with pin-receiving passages which are coaxial in one position of the plug and which receive pairs of mutually abutting pins, optionally having intermediate disc-like pin members, of which pins at least one of each pair of pins is spring biased towards the others, and the abutment surfaces of which pins when the correct key is inserted are located in the intersurface or dividing plane between the plug and the plug housing.
A person attempting to pick a cylinder lock of this kind can, for example, with the aid of a steel wire or like instrument, push up the pins, one after the other, in the plug to a position in said dividing plane between the plug and plug housing, said pins hereinafter being referred to as "underpins". When the plug is then turned slightly, the corresponding pin located in the housing catches on the cylindrical surface of the core, this pin being referred to hereinafter as the "upper pin". This pin has thus been forced or picked. The operation is then continued in a corresponding manner for each of the subsequent pins in the lock arrangement.
The upper pin can be lifted up and caught, as a result of the necessary clearance between the cylinder plug and the pins. When a pin is lifted up and caught, the pin passages are no longer co-axial.
The U.S. Pat. No. 1,593,513 (Stone) proposes that the mutually facing end surfaces of the upper and lower pins be given a frusto-conical configuration, in order to make picking of a cylinder lock in this way difficult.
The U.S. Pat. No. 2,111,098 (Segal) describes a development of this proposal. In the cylinder lock described in this Patent Specification, one or more of the plug-housing pins is provided in the region of its end facing the plug with a portion of smaller diameter than the remainder of the pin, this portion forming a neck between the major part of the pin and a head formed on the end thereof. The plug is also embraced by a sleeve of smaller wall thickness than the length of the neck portion, and is provided with a plurality of recesses or notches corresponding to the number of necked pins, these notches or recesses including a part which is co-axial with respective pin passages and one or more side portions of smaller dimensions.
Picking of such a cylinder lock is made difficult, because rotation of the plug for the purpose of hooking-up an upper pin can be effected with the use of said recessed sleeve, even when the limit surface between upper and lower pins is not located in the dividing plane between the plug and the housing. Consequently, anyone attempting to pick such a lock will wrongly assume that the first pin has been lifted up and caught, and will then continue with the subsequent pins, although the upper pin, however, is firmly held by the sleeve in a locking position in the plug. Consequently, anyone wishing to pick the lock must begin again with the first pin.
A particular advantage afforded by the arrangement described in said Patent Specification, is that the pin is firmly locked with the necked part inserted in a side part of the recess, so that the pin is unable to move upwardly or downwardly in the pin passage. Consequently, the plug is held locked relative to the housing, to prevent further turning of the plug, which is a prerequisite for preventing picking of the subsequent pins.
Although such an arrangement undoubtedly contributes to making the picking of such a lock difficult, it has the disadvantage that the metal sleeve embracing the plug increases the cost of the lock quite considerably, since in order to function reliably the sleeve must be made with great accuracy, and since the sleeve must also be mounted on the plug with great accuracy and precision.
The U.S. Pat. No. 2,283,489 (Crousore) illustrates another arrangement of the kind in question, in which a pin passage provided in the plug in line with a headed pin at a distance from the mouth of the passage, which mouth is smaller than the axial length of the necked part of the pin, exhibits a widened part which is bordered on one side by an edge of the mouth of a passage and which, subsequent to rotating the plug, is able to overlap a part of the pin head, to hook the pin in a locking position. The widened part is of eccentric shape, with its largest extension transverse of the plug axis.
Although this arrangement can be considered to render picking of the lock difficult, it has, among other things, the disadvantage that the lock cannot be used in a system lock of the kind in which intermediate, disc-like pin members are located between headed pins. Restrictions are particularly evident with respect to the thickness of the disc-like pin members. Consequently, it is impossible in practice to use thin disc-like pin members.
A corresponding disadvantage is found in the arrangement described in the U.S. Pat. No. 2,202,329 (Braune), in which arrangement the plug is provided with a longitudinally extending, rectilinear groove in the vicinity of the mouths of the pin passages. It is also impossible to use this kind of cylinder lock in system locks provided with disc-like pin members, since the pin members would slide uncontrollably in said groove.
The U.S. Pat. No. 3,762,193 (Hucknall) describe a different arrangement, in which the pins are not necked and headed, but which use instead conical disc-like pin members arranged to co-act with a constriction in the pin passages, to render picking of the lock difficult.